It's oscillator where shown in below with [impulse]. Oscillator sends an impulse to CPU when you click to this button.
First flip/flop gate set free a signal to right leg with first click and lamp turns on.
The first flip/flop gate switch power from right leg to left leg when Oscillator send second impulse and first lamp turns off and second lamp turns on.
This structure shown only how does only 1 flip/flop works. Actually this is not enough to shown 2 bits adder, to be honest we need one more flip/flpp gate
to understand next step. Please visit the 8 bits adder / CPU simulartor please for further detail.[impulse]